Digital processing of signals is usually more cost effective and provides higher performance than analog processing. Analog to digital (A/D) converters, therefore, are used in many systems. Many modern radio systems, for example, use digital signal processing techniques to achieve higher performance, and as such, require high performance analog to digital converters.
There are a number of different analog to digital (A/D) converters known in the art, such as flash A/D, successive approximation, pulse code modulation (PCM) conversion, oversampled PCM conversion, and sigma-delta modulation A/D conversion. Generally speaking, the best performance is achieved by sigma-delta modulation techniques.
Sigma-delta modulation A/D conversion achieves higher resolution than many of the other techniques. In essence, sigma-delta modulation improves the resolution by increasing the signal to noise ration (SNR) in the A/D converter, which results in increased resolution. The SNR increases due to oversampling, which spreads the noise over a large bandwidth, and due to a complex feedforward loop filter, which further reduces the noise in the band of interest by forcing some of the inband noise out of band. The extra performance of sigma-delta modulation A/D converters is obtained at the expense of high circuit speeds as compared to other A/D converters. This being the case, however, by using a complex transfer function for the complex feedforward loop filter the circuit speed requirements are less as compared to using a real transfer function. A complex sigma-delta modulation A/D converter uses such a complex filter.
Typical complex sigma-delta modulation A/D converter performance has been limited by mismatches in the circuit technology. In complex sigma-delta modulation A/D converters there are effectively two separate channels for the real and imaginary inputs to the complex feedforward loop filter. Any mismatch between the two channels causes the signals at the negative frequencies to `fold` into the desired signal band, which degrades performance. Therefore, and image frequency can be aliased on top of the desired signal.
Therefore, in order to exploit the high resolution and moderate circuit speed achieved by complex sigma-delta modulation A/D converters, a method and apparatus for minimizing the mismatch in the complex feedforward loop filter is needed.